Re: [PATCH 1/2] userfaultfd: remove one unnecessary warn_on in __mcopy_atomic_hugetlb

[Wei Yang <richardw.yang@linux.intel.com>]

On Wed, Sep 25, 2019 at 10:44:58AM -0700, Mike Kravetz wrote:
>On 9/25/19 5:18 AM, Wei Yang wrote:
>> The warning here is to make sure address(dst_addr) and length(len -
>> copied) are huge page size aligned.
>> 
>> While this is ensured by:
>> 
>>     dst_start and len is huge page size aligned
>>     dst_addr equals to dst_start and increase huge page size each time
>>     copied increase huge page size each time
>
>Can we also remove the following for the same reasons?
>
>diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
>index 640ff2bd9a69..f82d5ec698d8 100644
>--- a/mm/userfaultfd.c
>+++ b/mm/userfaultfd.c
>@@ -262,7 +262,6 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
> 		pte_t dst_pteval;
> 
> 		BUG_ON(dst_addr >= dst_start + len);
>-		VM_BUG_ON(dst_addr & ~huge_page_mask(h));
> 

Thanks for your comment.

It looks good, while I lack some knowledge between vma_hpagesize and
huge_page_mask().

If they are the same, why not use the same interface for all those checks in
this function?

> 		/*
> 		 * Serialize via hugetlb_fault_mutex
>
>-- 
>Mike Kravetz
>
>> 
>> This means this warning will never be triggered.
>> 
>> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
>> ---
>>  mm/userfaultfd.c | 4 ----
>>  1 file changed, 4 deletions(-)
>> 
>> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
>> index c7ae74ce5ff3..7895c715000e 100644
>> --- a/mm/userfaultfd.c
>> +++ b/mm/userfaultfd.c
>> @@ -243,10 +243,6 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
>>  		vm_shared = dst_vma->vm_flags & VM_SHARED;
>>  	}
>>  
>> -	if (WARN_ON(dst_addr & (vma_hpagesize - 1) ||
>> -		    (len - copied) & (vma_hpagesize - 1)))
>> -		goto out_unlock;
>> -
>>  	/*
>>  	 * If not shared, ensure the dst_vma has a anon_vma.
>>  	 */
>> 

-- 
Wei Yang
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